The Trophic Cascade

A young forest service employee named Aldo Leopold, charged with killing wolves in New Mexico in the early 1900s, started to notice that as the wolves died off, the deer population boomed and ate all the plants to nothing. In his groundbreaking piece “Thinking Like a Mountain,” from his book Sand County Almanac, Leopold put forth an idea 50 years ahead of his time: predators regulate ecosystems.

Since the reintroduction of wolves to Yellowstone National Park in 1995, we have learned much about the effect large carnivores have on an ecosystem. In the past, it was largely thought that an ecosystem was built from the bottom up, with plant life as the foundation from which everything else grew. Once healthy plants were established, insects, small rodents, birds, larger herbivores, and finally the top predators, all fell into balance with each other.

Almost all conservation and species reintroduction efforts were based on this theory. In a damaged area, biologists would first try to rebuild the plant life before doing anything else. However, some ecosystems could not be fixed before reintroducing an endangered top-level animal. In Yellowstone National Park, the U.S. Fish and Wildlife Service was required by the Endangered Species Act to reintroduce wolves before balancing the plant base and herbivore populations.

The Trophic Cascade

The result of the wolf reintroduction was an explosion of life and biodiversity as wolves returned to their natural place as apex predators. Scientists called this beneficial effect the “trophic cascade”, as wolves initiated a more natural ecosystem balance. Today, scientists have come from around the world to watch the effect wild wolves have on the park.

The idea of a “trophic cascade” is simple. The term “trophic” refers to the different levels of a food chain, with plants being one trophic level, insects the next, all the way up the ladder to herbivores and then predators. However, the “cascade” forces us to look at the traditional food chain from a different perspective. Wolves, as top predators, have a cascading beneficial effect on all the trophic levels that are below them.

Wolves and Elk

Wolves have this beneficial Trophic Cascade effect for one simple reason: They make elk run.

Consider what elk do when left alone. Entire herds like to go to riversides and eat everything they can find in one place (grasses, bushes, saplings, even small trees) before moving onto another and doing the same thing. After decades without wolf predation, elk had denuded Yellowstone’s landscape and killed many of the smaller trees that line riverbanks.

When wolves were reintroduced, the elk herds could no longer sit in one place and eat everything nearby. They were forced to keep moving in response to wolf predation.

This had myriad benefits. When wolves chase elk during a hunt, the elk are forced to run faster and farther. As the elk run, their hooves aerate the soil, making it prime for water retention and allowing more grasses to grow.

Since the elk cannot remain stationary for too long, aspens and willows in one area are not heavily grazed, and can therefore fully recover between migrations. This meant that songbirds could come back in droves to nest in the new habitat. The trees even shade the rivers, making the water colder. Colder water can hold more oxygen, which provided a boost to fish populations.

Coyotes and Bears

Wolves perform one other essential ecosystem service: They keep coyote populations in check.

As with the rest of the country, coyote populations were nearly out of control in Yellowstone before the wolves returned. Now, the coyotes have been out-competed and essentially reduced by nearly 80% in areas occupied by wolves. The coyotes that do remain are more skittish and wary. With fewer coyotes hunting small rodents, raptors like the eagle, hawk, and osprey have more prey and are making a comeback.

Grizzly bears also benefit from wolf presence: The endangered grizzly bears successfully steal wolf kills more often than not and thus have more food to feed their cubs.

The grizzly bears also benefit from the vegetation regrowth, and in turn, as top predators, help reinforce the effect of the wolves on prey species. In essence, we have learned that by starting recovery at the top of the food chain, predators like wolves create benefits for the whole ecosystem. A wild wolf population actually makes for a stronger, healthier, and more balanced ecosystem. From plant, to insect, to people — we all stand to benefit from wolves.

With only 5% of our nation’s wilderness left, people are recognizing the importance of complete ecosystems in keeping all of us healthy. With new knowledge of trophic cascades, we can now begin to focus wilderness recovery efforts on a wider variety of ecosystems. Using Yellowstone as an example, we can teach the world about the wolf’s positive and vital role in nature.